A sensing machine of this type is known from DOS No. 2316993 according to which, for the purpose of exactly positioning a corner stitch in a seam or seam portion, the actual feed of the workpiece is determined and an intervention takes place in the material transfer in the seam area prior to a corner stitch for the purpose of varying the length of one or more stitches. Quite apart from the complicated and therefore expensive mechanisms involved, e.g. in the form of a motor adjustment of the stitch regulating lever, there is also a modification to the action conditions of the feed dog on the workpiece, so that the complete stitch length adjustment is subject to tolerances. In addition, such a construction does not permit any modification in a very short time. The modification of the action conditions by varying the size of the thrust components of a feed dog performing a quadrangular movement, only makes it possible to inadequately calculate beforehand stitch length changes. This is particularly difficult if consideration is given to the processing of widely differing materials, such as e.g. jersey material on the one hand and cotton on the other, because in both cases action on the material feed leads to completely different results, so that subsequent corrections are constantly necessary for obtaining adequate results.